Measurement and implications of the 
s/
t
Hypoxemia can be caused by a variety of factors (Box 32-1). This chapter will focus on the primary cause of hypoxemia—shunting, which may, at times, be due to ventilation/perfusion (
/
) mismatch.
Ventilation/perfusion mismatch
Ideally, pulmonary perfusion (
) evenly matches alveolar ventilation at all levels of the lung; however, perfect matching does not occur because the distribution of ventilation and perfusion and the 
/
ratio vary throughout the lung. A normal lung has a 
/
ratio of approximately 0.8. A 
/
ratio of 0 (i.e., a shunt) exists when perfused alveoli have no ventilation and the values for PO2 and PCO2 of the trapped air are the same as those of mixed venous blood (PO2 = 40 mm Hg and PCO2 = 47 mm Hg). Conversely, a 
/
ratio of ∞ exists when ventilated alveoli have no perfusion and, at sea level, the PO2 and PCO2 equal approximately 150 and 0 mm Hg, respectively. Nonperfused alveoli (i.e., alveolar dead space) is approximately 25 to 50 mL in a healthy 70-kg person. Figure 32-1 depicts the progression of a 
/
ratio from 0 to ∞; the normal, idealized, alveolar-capillary unit is shown as example A.
